British scientists' exploration of the ocean depths has revealed an amazing
array of new lifeforms, at depths where life was once thought impossible.

In 1841, the Royal Navy survey ship HMS Beacon was ordered to chart the waters of the Mediterranean and Aegean. On board was a naturalist called Edward Forbes, who took the opportunity to dredge up samples of ocean life from depths never before sampled.

He returned home almost empty-handed and was forced to conclude that life could not exist below 300 fathoms (540 metres). He named this barren region of the sea the "Azoic" – meaning lifeless – zone.

For a century, this view remained unchallenged. Yet even though the depths of the oceans remain one of the least chartered regions of the planet, scientists are discovering that a host of bizarre creatures survives down in the depths.

Research by a British team recently returned from the Pacific trenches near Japan has revealed fish that create their own light, or have enormous mouths and terrifying teeth, as well as hairy snails, clams the size of dinner plates, and shrimps with eyes on their backs.

The reason scientists believed for so long that life did not exist in the deep sea is because conditions are extraordinarily tough. The oxygen that filters down is centuries old, having formed near the surface through photosynthesis by microscopic plants known as phytoplankton.

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In the "hadal" zone, which at 11,000m is deeper than Mount Everest is high – the pressure rises to 1,000 bar, or a ton per square centimetre. And as there is practically no light, plants cannot grow, so there is little food.

Instead, a small amount of nutrition filters down to the bottom as a rain of small particles, or in the form of dead fish that sink. "It's like being in a desert," says Professor Julian Partridge of the University of Bristol. "There's nothing to eat, so the density of animals is low."

With so few creatures around, finding a mate is as much of a problem as finding food. Some animals have come up with ingenious solutions: when the male deep-sea angler fish bumps into a female, he uses his lips to remain attached to her as a parasite. "It's a kiss that lasts the rest of his life," says Dr John Copley, a deep-sea biologist from Southampton's National Oceanography Centre. "After that he no longer feeds, but hooks up to her blood supply."

Somehow, however, the ocean still manages to sustain an astonishingly diverse range of life. "There are more species of animal in the deep sea than beetles in the rainforest," says Dr Copley.

The Census for Marine Life, a global network of researchers in more than 80 nations, estimates that about 230,000 species of marine animals have been described, and that there could be a total of between 500,000 and 10 million species in the sea.

Attempting to catch these creatures, though, is difficult. "Trying to find animals in such a vast volume has been likened to towing a net across a field from a helicopter that's flying through cloud," says Prof Partridge. "What would you catch? The answer is the slow and the stupid."

There are also logistical difficulties: "If you trawl at 4,000m, scraping the sea floor, you'd need a wire that's eight miles long. That means when you're sitting on a research ship, the net is actually over the horizon."

A group of Scottish scientists, however, has come up with an alternative approach. At the end of March, they returned from a research trip to the deepest zone on the planet: the long, narrow and extremely deep trenches on the edge of the Pacific Ocean, formed when tectonic plates collided.

The project, a collaboration with the University of Tokyo known as HADEEP, uses an autonomous, free-falling craft or "lander" capable of recording video footage. The team had to design a window able to withstand the equivalent pressure of 1,600 elephants standing on the roof of a Mini: instead of glass, it is made of a sheet of sapphire.

In July 2007, the HADEEP team sent this lander to a depth of 10,000m in the Tonga trench between Samoa and New Zealand in the South Pacific.

They were astonished at what they found: the largest group of fish ever seen in the hadal zone. "At one time there were 19 rat-tail fish in the frame," says Dr Alan Jamieson, from Aberdeen University. "Another high point was the snail fish, which is pink and slimy, with tiny black eyes and an orange blob that are its internal organs."

This year the team returned to the hadal zone, sampling the Izu-bonin trench off the coast of Japan. Instead of fish, they found massive numbers of ugly creatures known as "amphipods", similar to shrimps. "The traps were so full you couldn't fit another one in if you tried. Some were laden with eggs, which was enough to put you off your dinner," says Dr Jamieson.

The team believes that the trenches they were exploring are almost like inverted islands – species become trapped within them, unable to venture to depths to which they have not adapted, and hence evolve in a subtly different way from those found in other trenches, just like the creatures of the Galapagos. "Like Darwin's finches, over time, they end up slightly differently from one another."

The HADEEP project is not the only one searching out bizarre new creatures. Other recent discoveries include the barreleye, or spookfish, which has eyes that pivot through 90 degrees, and a transparent skull, in which its coral-reef-like brain glows eerily.

Prof Partridge recently found a brown-snout spookfish at 800m above a trench in Tonga. It looks as if it has four eyes, but in fact each of the two eyes is divided, one section pointing up and the other down and to the side. What is truly extraordinary, however, is that the fish uses a mirror to focus light on to its retina, created using tiny, carefully stacked plates of a protein called guanine.

"No other vertebrate has been known to do this," says Prof Partridge. "We've been using lenses to form an image for more than 400 million years. But this system gives the fish a field of view below and to its side using a mirror – that's a wonderful example of evolution finding novel solutions to problems."

Then there is the black dragon fish, which emits a red light that approaches the infra-red wavelength. By using a chemical from bacteria to modify its light-sensing cells, the creatures are able to see in this spectrum: recently, a researcher from Columbia University in New York has injected the same chemical into mice, creating rodents that can also see this far-red light.

As Prof Partridge says: "Who knows what other animals are down there with weird and wonderful attributes about which we know nothing?"